The liver is an extraordinary organ capable of modulating its mass according to functional requirements, proliferating under conditions of functional deficiency, and undergoing apoptosis under functional excess [1]. Upon surgical removal of two-thirds of tissue compensatory growth of the remaining portion is observed in the liver to restore the resected mass, although without anatomical reconstitution [2]. While stem or progenitor cells present in postnatal tissue may contribute to the regeneration of liver, the process takes place without the dependence of such cells. Serial repeated partial hepatectomies performed on rodent liver demonstrated restoration of resected tissue mass in the absence of apparent oval cell proliferation [3]. Further, transplantation of a small number of hepatocytes into urokinase-type plasminogen activator transgenic mice resulted in complete repopulation of the liver [4]. These findings demonstrate that hepatocytes possess proliferative potentials under in vivo environments.
Many patients suffer from liver dysfunctions and diseases and, in the shortage of organ donors, there is an increasing clinical demand for hepatocytes for transplantation-based therapy. Although hepatocytes have been shown to exhibit great replicative capacity in vivo, it has been difficult to obtain primary cultures of hepatocytes that both proliferate and maintain liver-specific functions in vitro [5, 6]. Long-term primary cultures of hepatocytes from various mammalian species have been studied extensively over the past three decades and, while improvements in culture conditions have been made to sustain characteristic hepatic functions, cultured hepatocytes show little replicative capacity in vitro [7-16]. It was not until more recently that Hino et al. and Katsura et al. reported conditions enhancing the in vitro proliferative potential of human hepatocytes while retaining differentiated phenotypes [17, 18]. Nevertheless, it is difficult to obtain large numbers of human hepatocytes for clinical applications.
Somatic stem cells, such as mesenchymal stem cells, are multipotent stem cells capable of differentiating into various lineages of the mesoderm, and are easily accessible from bone marrow, umbilical cord blood, and numerous other postnatal tissues [19-23]. It has been previously demonstrated that somatic stem cells isolated from human bone marrow and umbilical cord blood can differentiate into hepatocyte-like cells with morphology, gene expression, and in vitro functions characteristic of hepatocytes [20, 24, 25]. Albeit the differentiated cells exhibit a number of hepatic characteristics, these cells do not possess and sustain a complete repertoire of the properties of parenchymal liver cells, indicating that the differentiation process is only partial.
A previously unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies, especially in connections with culturing hepatocytes.